1. Field of the Invention
This invention relates to processes that reduce the effect of baseline wander and intersymbol interference in signals such as those used in communications.
2. Description of Related Art
A modem or other transmitter using pulse code modulation (PCM) to transmit data generates a signal that includes a series of pulses representing a series of symbols. Each pulse has an amplitude that indicates a value for a corresponding symbol. A receiver determines the symbol values by measuring the amplitudes of the pulses relative to a baseline voltage. However, when the pulses are sent through a channel that blocks or attenuates DC or low frequency signal components, the level of each pulse sags toward zero during the duration of the pulse. This sag affects the level of the next pulse. In particular, the received amplitude of the next pulse is larger if the next pulse is of opposite polarity from the preceding pulse, and the amplitude of the next pulse is smaller if the next pulse is of the same polarity as the preceding pulse. This inter-symbol interference changes the baseline for accurate determination of pulse amplitudes and is commonly referred to as baseline wander. Similar baseline wander occurs in signals using other modulation protocols common to many standard communication signals. Baseline wander can cause a receiver to incorrectly identify symbol values associated with a portion of a received signal.
A transmitter can reduce baseline wander by reducing or avoiding the low frequency components in the transmitted signal. However, avoiding low frequency components reduces the bandwidth available for information transmission. Alternatively, a receiver can compensate for baseline wander using a feedback mechanism similar to the one described in "Quantized Feedback in an Experimental 280Mb/s Digital Repeater for Coaxial Transmission," by F. D. Waldhauer, IEEE Trans. Communications, Vol. COM-22, No. 1, January 1974, pp 1-5. These feedback mechanisms commonly assume a particular model for the characteristics of the transmission channel and have little or no ability to adjust for actual performance of the channel. Thus, improved systems for handling baseline wander are needed.